Re: JNI Unicode String puzzle

Zig <>
Thu, 20 Dec 2007 02:40:04 -0500
On Tue, 18 Dec 2007 00:38:46 -0500, Roedy Green =

<> wrote:

On Tue, 18 Dec 2007 01:37:24 GMT, Roedy Green
<> wrote, quoted or indirectly quoted
someone who said :

If you do JNI GetStringChars in C++, just what do you get? an array
of TCHARS? A null terminated TCHAR string?

I'll assume you're writing for Windows.

Mystery solved.

GetStringChars (16-bit) does not terminate with null. You must use
wcsncpy_s to provide one.

GetStringUTFChars (8-bit) does terminate with null.

In one of my standard includes for my Windows JNI projects, I have a =

protype for the function:

LPWSTR GetSzwStringCharsFromHeap(JNIEnv * env, HANDLE hHeap, jstring jst=
    LPWSTR lpwResult=NULL;
    jsize jStrLen;

    if (jstr==NULL)
        goto finished;
    jStrLen=(*env)->GetStringLength(env, jstr);

    lpwResult=HeapAlloc(hHeap, HEAP_ZERO_MEMORY, (jStrLen+1)*sizeof(WCHAR=
    if (lpwResult==NULL)
        fireJavaExceptionForSystemErrorCode(env, GetLastError());
        goto finished;
    (*env)->GetStringRegion(env, jstr, 0L, jStrLen, lpwResult);

    return lpwResult;

(Callers should use (*env)->ExceptionCheck(env) to see if this function =

actually succeeded).

If there is a more conventional approach, I'ld love to hear it. Using =

GetStringRegion to copy data to the native buffer once seems like it =

should be more efficient than allocating a non-terminated buffer and a =

terminated buffer.

C++ Unicode 16-bit functions do not work (quietly degrade to 8-bit
mode) unless you define BOTH:

#define UNICODE
#define _UNICODE

I try to avoid using LPTSTR and TCHAR wherever possible, and instead fav=
or =

LPWSTR and WCHAR. Most Windows functions are declared as

#ifdef UNICODE
#define SomeFunction SomeFunctionW
#define SomeFunction SomeFunctionA

(With the exception that functions new for Vista / Windows 2008 are =

generally UNICODE only)

Thus, I explicitly call SomeFunctionW, thus avoiding the compiler's glob=
al =

UNICODE definitions.

Isn't the UNICODE declaration supposed to be set by the C compiler's =

environment when it's in Unicode mode (which to me would suggest the =

compiler will compile "xyz" the same as L"xyz")? Since <jni.h> expects =

method & type signatures to be supplied as char* , it seems like switchi=
ng =

the compiler to the full-blown Unicode mode would then break when you =

attempt to make JNI calls of the form:

(*env)->FindClass(env, "java/lang/Object");

Anyway, as some of this is speculation and my experimentation with such =

settings is minimal, I'ld be curious how your mileage goes.

For what it's worth though, if you just use the "W" functions and avoid =

the TCHAR abstraction, the rest seems to fall into place.

I had forgotten what a nightmare C++ deeply nested typedefs with a
dozen aliases for every actual type are. YUCCH!

It came clear with sizeof dumps.

Hope that was interesting or useful,


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